This is used to 'rescue' legitimate bounce messages that were generated in response to mail you really *did* send. List the MTA relays that your outbound mail is delivered through. If a bounce message is found, and it contains one of these hostnames in a 'Received' header, it will not be marked as a blowback virus-bounce.

The hostnames can be file-glob-style patterns, so relay*.isp.com will work. Specifically, * and ? are allowed, but all other metacharacters are not. Regular expressions are not used for security reasons.

Multiple addresses per line, separated by spaces, is OK. Multiple whitelist_bounce_relays lines are also OK.